1. Field of the Invention
The present invention concerns an altimetry method performed from the air or space, especially ocean, altimetry.
The present invention also concerns a device for implementing the method.
2. Description of the Prior Art
There are many methods for carrying out ocean altimetry. The three main parameters associated with these methods and defining their respective performance are: vertical precision, spatial resolution and swath.
Most conventional methods are based on the use of radar emitting signals of suitable wavelength, for example airborne radar. There are various types of radar: single-pulse system, interferometric system, etc. Altimetry of this type has been mostly limited to nadir-looking type instruments, restricting the range of possibilities.
Some prior art methods use existing radio frequency signals and combine in an airborne or space receiver signals from the transmitter (direct signals) and signals reflected from the ocean (or, more generally, from the terrestrial surface). In the following description the expression xe2x80x9cterrestrial surfacexe2x80x9d refers to the terrestrial crust, or to the surface of oceans, seas or lakes, or to the surface of frozen water.
Usable existing sources of radio frequency signals, known as xe2x80x9csources of opportunityxe2x80x9d, include communication, television and navigation satellites systems such as the xe2x80x9cGPSxe2x80x9d (xe2x80x9cGlobal Positioning Satellitexe2x80x9d), a satellite navigation system used in the West and its equivalent GLONASS in the former Soviet Union or the future European Galileo system. The navigation signals from a Global Navigation Satellite System (GNSS) are particularly suited for altimetry application.
The patent U.S. Pat. No. 5,546,087 discloses an altimetry method with a specific application to estimating the height of seas of oceans and variations therein.
This method uses the amplitude of multiple correlations of the direct and reflected signals. However, the vertical accuracy obtained by this method is limited to the bandwidth of the opportunity signals. A better precision can be achieved by combining measurements, resulting in a loss in spatial resolution.
An object of the present invention is to improve the vertical accuracy obtained in the measurement of height variations of the ocean surface using GNSS signals in order to achieve an accuracy in the mean sea level below 10 cm.
The present invention: provides a method for performing Earth altimetry comprising the steps of:
receiving by an upward-looking antenna onboard a platform above the Earth surface, direct signals having at least two different carrier frequencies transmitted by GNSS satellites in view of said upward-looking antenna,
receiving by a downward-looking antenna onboard said platform signals reflected by the Earth surface and having said at least two different carrier frequencies,
comparing carrier phases of said direct signals and received reflected signals, at said carrier frequencies, and determining from said phase comparisons a surface height.
According to an aspect of the present invention, the method further comprises a step of processing said direct signals to provide time, position and velocity of the phase center and clock of a GNSS receiver.
According to another aspect of the present invention, the method further comprises receiving direct and reflected signals having three different carrier frequencies.
Another object of the present invention is a device for performing Earth altimetry with an accuracy in the mean level below 10 cm. This device comprises:
an upward-looking antenna onboard a platform, for receiving direct signals having at least two carrier frequencies from GNSS satellites in view,
a downward-looking antenna onboard said platform for receiving signals at said carrier frequencies, these signals being transmitted by each of said GNSS satellites and reflected by the Earth surface,
a GNSS receiver connected to said upward-looking antenna for providing direct signals having said carrier frequencies,
a frequency de-multiplexer connected to said downward-looking antenna for separating said received reflected signals into reflected signals having respectively said carrier frequencies,
means for comparing carrier phases of said direct signals outputted by said GNSS receiver and reflected signals outputted by said frequency de-multiplexer, at said carrier frequencies, and means for determining a surface height from said phase comparisons.
For simplicity, but without any intention of limiting the invention, the remainder of this description concerns signals from the GPS satellite system and a receiver according to the present invention, which is onboard a satellite in Low Earth Orbit (xe2x80x9cLEOxe2x80x9d). This receiver can be also an airborne receiver without departing from the scope of the invention.
Also for reasons of simplicity, unless otherwise indicated the remainder of this description concerns only specular reflection. Signals obtained by diffuse reflection can nevertheless be used, especially in sea ice mapping applications.
The invention will be more clearly understood and other features and advantages of the invention will emerge from a reading of the following description given with reference to the appended drawings.